The present invention relates to communication systems, and more specifically to frequency error correction in a communication system.
In a conventional radio communication system, a transmitter sends information over a high frequency channel. Typically, a radio frequency upconverter converts a baseband signal to a high frequency signal prior to its transmission. A baseband signal can be upconverted in frequency by a multiplication process. This signal is mixed by the output signal of a local crystal oscillator that is tuned to a modulating frequency. The output signal of the mixer is the baseband signal shifted in frequency by an amount corresponding to the modulating frequency of the local crystal oscillator. A receiver receiving the transmitted signal first down converts the signal by shifting it back to a baseband frequency range. A local oscillator tuned to the modulating frequency is multiplied by the transmitted signal.
A cellular communication system is one type of radio communication system mentioned above, in which radio telephones contain circuitry to permit transmission and reception of modulated signals. The radio telephones, known as mobile stations, engage in a two-way communication with remotely located transceivers, known as base stations. These radio telephones also have a local crystal oscillator for down converting into baseband range, the signals that the base station transmits.
One problem with the radio communication arrangement described above is that the crystal oscillator frequency at the receiving station may not be exactly the same as the modulating frequency at the transmitter. This frequency offset between the transmitter and the receiver stations may cause communication errors. One of the possible reasons for this frequency offset is that the output frequency signal of the local oscillator may drift away from the intended modulating frequency over a period of time due to, for example, the ambient temperature variations at the receiver. Another possible reason for this frequency offset may be relative motion between a base and a mobile station. As it is well known, such relative motion between a transmitting and a receiving station, may result in a Doppler shift of the frequency of the signal being transmitted. This Doppler shift may therefore result in a phase/frequency error in the received signal, for example. Because of the ambient temperature variations at the receiving station and the Doppler shift described herein, for example, the integrity of the signal being transmitted may be corrupted at the receiving end of the communications system.
A need therefore exists for an adaptive frequency correction system that is able to acquire and track the relative frequency error between a mobile station and a base station.
In accordance with one embodiment of the invention, a method for reducing frequency offset errors in a communications system comprises the steps of receiving a plurality of signal bursts; correcting the frequency error with the coarse frequency error estimate; decoding the received signal bursts by employing an equalizer, that generates a burst quality indication signal corresponding to the bit error rate in each received signal sample; generating a plurality of residual frequency offset signals corresponding to each signal burst; channel decoding said signal bursts generated by said equalizer so as to generate a channel quality signal repeating the bit error rate over a plurality of burst signals; accumulating residual frequency offset signals corresponding to bursts having acceptable burst quality indication signals and channel quality indication signals; acquiring a rotation frequency offset signal based on the accumulated residual frequency offset signals; and adjusting the phase of the received signal samples in accordance with the rotation frequency offset signal.